The reported technology for growing directionally oriented cast structures from superalloys has evolved from processes suitable for making simple shapes and members to processes that are currently used to form articles having complex shapes, such as the directional solidification of Ni-base superalloy blading members used in the hot sections of gas turbine engines. The published literature, such as Metals Handbook Ninth Edition, Volume 15 Casting, ASM International (1988), pp. 319-323, has many examples of processes for making directionally oriented, superalloy blading members, such as turbine blades and vanes. Most of these processes utilize some form of a withdrawal-type vacuum induction casting furnace with mold susceptor heating.
In the art of casting to produce directionally oriented superalloys, fluid pressure, such as an inert gas or air, has been applied within a closed container to a molten material, such as a metal, to force the molten material upwardly through a tube. A patent which discloses one such method and associated apparatus is U.S. Pat. No. 3,302,252, relating to continuous casting of an article upwardly through a pouring tube into a cooled mold. The cast article is continuously withdrawn from the mold.
Another portion of the casting art sometimes is referred to as the EFG (Edge-defined, Film-fed Growth) process. In that process, no external pressure is applied to a liquid material, but capillary action within a narrow forming tube or die is relied upon to draw the liquid material upwardly for solidification. Frequently, a seed crystal is introduced into the liquid to initiate crystal growth. Typical patents which disclose features of this process include U.S. Pat. Nos. 3,471,266; 4,120,742 and 4,937,053.
In some of the above referenced patents and elsewhere in the casting art relating to the formation of directionally solidified or single crystal articles, seed crystals having selected crystal orientations (primary and/or secondary orientations) have been used. They constitute a means for initiating the solidification of an article having a desired crystal orientation. In the formation of blading members, the seed crystals are also used in conjunction with casting forms, such as ceramic molds, to define the shape and crystal orientation of the member.
Heretofore, the joining of components of single crystal or directionally solidified elongated grain articles, including turbomachinery airfoils, has generally involved the use of separately cast members of selected crystal orientation. Such members are assembled and bonded into an article across an interface between the members. U.S. Pat. Nos. 3,967,355 and 4,033,792 are representative of patents relating to this type of bonding, and the '792 patent describes the desirability of matching crystal structures across the bond interface.
By using the casting technology described above, a directionally oriented article, such as a blading member, can be formed as a single crystal or with a directionally solidified crystal structure comprising a plurality of columnar grains. Both single crystal and directionally solidified articles may be formed with preferred crystal orientations, and these orientations may be formed within components so as to produce non-isotropic, orientation-related physical and mechanical properties along certain directions within the component. The desired crystal orientation in nickel-base superalloys frequently used for turbine engine components, such as blading members, is that the &lt;001&gt;crystallographic direction be parallel to the longitudinal axis of the member, in order to minimize the elastic modulus along the length of the member. This orientation is known to provide a good balance of the creep strength, ductility and thermal fatigue resistance of these components. Thus, these members are formed, as described herein, so that the &lt;001&gt;direction is the growth direction and corresponds to the longitudinal axis of the member.
An example of a blading member having a complex shape of the type described above is the turboomachinery blade described in U.S. Pat. No. 4,010,531. Such a blading member comprises an airfoil-shaped outer wall having a complex hollow interior communicating with an end region, such that gases can be circulated from the hollow interior through the outer wall and end region for cooling purposes, wherein the end region comprises a tip that extends from the end of the member.
Airfoil blading members, and other gas turbine engine components, are frequently utilized in extreme environments where they are exposed to a variety of environmentally related damage and wear mechanisms, including: erosion due to impact by high-velocity and/or high temperature airborne particles, high temperature oxidizing and/or corrosive gases, low-cycle fatigue processes and mechanical abrasion caused by rubbing against other members. These mechanisms are known to cause cracking and other damage, particularly in the end regions or tips of the blading members. Because the manufacturing costs for blading members are typically relatively high, it is often desirable to repair rather than to replace them after the tips have been damaged or worn. When superalloy blading members, or other superalloy articles having a directionally oriented microstructure, are damaged in the tip or extended end region, whether in operation or during manufacturing, the problem of their repair becomes more complicated and difficult, because of the necessity of maintaining physical and mechanical properties in the repaired portion that do not degrade the overall performance of the component. This problem of repair becomes particularly acute when a directionally oriented microstructure must be maintained in the repaired portion, as is frequently desirable in directionally oriented articles such as airfoils, because of the difficulty of replicating the original directional orientation in the materials used to make the repairs.
One method that has been used for the repair of turbine blade tips, has been to add material to the damaged or worn portion of the tip by welding, or similar processes. A disadvantage of this method is that the microstructure of the weld is not directionally oriented, and thus the mechanical properties of the tip or extension are diminished as compared to the remainder of the directionally oriented microstructure of the article. Also, most current oxidation resistant materials are difficult to weld, and have been known to crack during the welding process.
Another method has been to add separately formed tips to the end of an airfoil by brazing, welding, diffusion bonding or similar bonding processes. This method is described, for example, in U.S. Pat. Nos. 3,967,355, 4,010,531 and 4,033,792. Using such methods, it is sometimes desirable to form a crystal structure in the tip that is similar to that of the remainder of the airfoil, and to develop a microstructure in the bond that is compatible with the microstructures of both the tip and the remainder of the airfoil.
U.S. Pat. Nos. 5,291,937 and 5,304,039, which are both assigned to the assignee of this invention and are hereby incorporated by reference herein, also describe two methods for providing an extension on the end of a directionally solidified article, such as a blading member. These methods both utilize a die and a die extension made from ceramic materials, and involve applying a fluid pressure to force a molten material into the die extension. The article end on which the extension is to be formed is then placed into the die opening and die extension and into contact with the molten material. The article end is held in contact with the molten material for a time sufficient for the article end to interact with the molten material, whereupon the article is withdrawn through the die opening at a rate that permits directional solidification of an extension on the end of the article. A description is given of how these methods may be used to repair blading members, particularly their end regions and extended tips.
However, it is desirable to develop other methods of providing extensions on the ends of directionally solidified articles, such as a blading members, particularly methods that do not require the apparatus described in the referenced patents, such as the ceramic die and die extension, and the means for applying fluid pressure to force the molten material into the die.